Nuclear fusion occurs when two relatively light atomic nuclei (e.g., isotopes of hydrogen, helium or lithium) are brought into such close proximity that they fuse Into a single heavier nucleus, releasing tremendous amounts of energy in the process. For over half a century, the theoretical potential of nuclear fusion as a clean, reliable, and virtually inexhaustible energy source has been known and has motivated an array of research and development projects.
Practical fusion technology, however, remains elusive. Fusing two nuclei requires confinement; i.e., the nuclei must be held in very close proximity to each other for a period of time sufficient to allow the fusion reaction to occur. Confinement requires overcoming the Coulomb barrier that causes the positively charged nuclei to repel each other. The most common approach to overcoming this barrier involves directing the nuclei toward each other with sufficient momentum to penetrate the Coulomb barrier and achieve confinement.
Over the years, various techniques have been tried for imparting the necessary momentum to the nuclei. For instance, inertial confinement fusion (ICF) is being investigated at various research centers, including the National Ignition Facility (NIF). In ICF, the fusion fuel (typically a deuterium-tritium mixture) is placed within a spherical capsule that has a thin outer shell (called an ablator). An inner shell made of the fusion fuel in a solid or liquid state usually lines the inner wall of the ablator, and the interior of the inner shell is filled with a low-pressure gas of the fusion fuel. When heated, the ablator rapidly expands outward, driving the inner shell inward and compressing the fuel. Under the right conditions, the compressed fuel forms a central “hot spot/” containing 2-5% of the fuel, in which confinement is attained. Heat released from the resulting fusion reactions in the hot spot, then radiates outward to create an expanding thermonuclear burn front.
Heating of the ablator can be done directly or indirectly. In “direct drive” ICF, a conventional energy source, such as a laser or ion beam, is directed onto the capsule surface to heat and expand the ablator material, driving an implosion of the fuel. This approach demands very uniform illumination of the capsule surface to avoid hydrodynamic instability that would preclude confinement or the development of a sustained burn front. In “indirect drive” ICF, the fuel-containing capsule is placed in a “hohlraum,” a symmetric cavity with walls made of a high-Z material such as gold, lead, or uranium that acts as a blackbody radiator. Laser or ion beams are directed onto the walls of the hohlraum, which radiates x-rays into the cavity. The x-rays heat and expand the ablator material, driving an implosion of the fuel. The use of a hohlraum reduces sensitivity to hydrodynamic instability, resulting in relaxed requirements for uniform illumination. Nevertheless, a symmetric implosion of the fuel is crucial.
Thus, capsule design is an important factor in ICF. For example, inner shells with a large radius and small thickness achieve high implosion velocities. In addition, nonuniformities in the ablator, and to a lesser extent in the inner shell, can result in asymmetry in the implosion so that confinement does not occur.
Various capsule dimensions and compositions have been proposed and studied. For example, one existing capsule design provides a plastic (CH) ablator with an outer radius of about 1.1 millimeters (mm) and a thickness of about 0.15 mm. The inner shell was made of solid deuterium-tritium (DT) ice about 80 micrometers (μm) thick; the interior was filled with DT gas at a pressure of 0.3 mg/cm3 at a temperature of about 4 K. Another capsule had similar dimensions, but the ablator was made of beryllium doped with sodium and bromine. Other capsule designs use glass or silicon dioxide microballoons with diameters on the order of 150 and wall thicknesses on the order of 5-10 μm as ablators.
In practice, existing capsules have generally not produced satisfactory results. Typical problems include nonuniformity in the ablator thickness or composition, as well as deviations from sphericity and defects in the surface finish of the ablator. Any of these problems can lead to asymmetry in the implosion of the fuel. In addition, while a relatively thin ablator is generally desirable, the ablator needs to be thick enough to resist the pressure of the fuel inside, and the strength of the ablator material can be a limiting factor on the density of the fuel.
It would therefore be desirable to provide an improved fuel capsule for an ICF reactor.